1. Field of the Invention
The present invention relates to a circuit for changing a frequency having a cascade including a transadmittance with a voltage-current transfer characteristic and a transimpedance with a current-voltage transfer characteristic, wherein the transadmittance has two terminals for a signal of a first frequency and the transimpedance has two terminals for a signal of a second frequency.
2. Description of the Background Art
A circuit is known from the publication “Bipolar High-Gain Limiting Amplifier IC for Optical-Fiber Receivers Operating up to 4 Gbit/s,” IEEE Journal of Solid State Physics, Vol. sc-22, No. 4, August, 1987.
In this context, a transadmittance is generally understood to be a voltage-to-current converter, and a transimpedance is generally understood to be a current-to-voltage converter. For reasons of cost, communication systems in the future are expected to use what are known as “one-chip” solutions which integrate a power amplifier (PA) in addition to the transmit and receive path. In such an integrated arrangement, interactions occur between the power amplifier and the voltage-controlled oscillator (VCO). In many arrangements, the VCO oscillates at the transmit frequency. In transmit operation, the output signal of the power amplifier then couples to the VCO with maximum level and detunes it. The degree of undesirable coupling increases with the power of the power amplifier, sharply limiting the output power of one-chip transceivers (transmitter/receivers).
In other arrangements the VCO oscillates at half the transmit frequency. The frequency of the output signal is then doubled. A disadvantage is that frequency doublers either have only asymmetrical output signals, or, when mixers are used, must have exact phase relationships, which vary considerably with manufacturing dispersion. Asymmetrical output signals lead to a mismatch at the output with subharmonics at half the frequency of the power amplifier, thus at the frequency of the voltage-controlled oscillator. The subharmonics interfere with the signal from the voltage-controlled oscillator. Moreover, in this concept only low reference frequencies are possible in the phase-locked loop (PLL) of the voltage-controlled oscillator, resulting in long lock times and thus long settling periods for the phase-locked loop.
It is also known to have the VCO oscillate at twice the transmit frequency. An interfering coupling of the frequency of the power amplifier into the voltage-controlled oscillator then occurs at the first harmonic (twice the frequency) of the signal of the power amplifier. The undesirable coupling is thus merely shifted to higher frequencies, where the harmonics of the power amplifier already exhibit a significant decrease in power. A disadvantage is that the coupling is merely reduced, and the maximum power thus is still limited to low values.
The use of a frequency offset between the VCO and the power amplifier is also known. Considerable effort with additional mixers and VCOs is necessary here. See also M. H. Norris, “The Design of Digital Cellular Handsets,” IEEE Colloquium, pp. 4/1-4/6, March 1998.